1. Field of the Invention
The present invention relates to an image forming apparatus with an improved print quality such as a copier, a printer, a facsimile machine or a complex machine of these.
2. Description of the Related Art
Conventionally, there has been known the following electrophotographic image forming apparatus. An electrostatic latent image is formed by exposing a surface of a photoconductive drum including a photoconductive layer made of OPC, amorphous silicon or the like and uniformly charged by a charger with light from a laser, an LED or the like in accordance with image information. This electrostatic latent image is developed into a toner image by a developing unit and this toner image is transferred to a transfer medium (sheet) by a transfer unit. The transfer medium is separated from the photoconductive drum by a separator and the toner image on the transfer medium is fixed to the transfer medium by a fixing device to output an image.
In such an image forming apparatus, a bias power supply is disposed to apply a bias voltage to a transfer roller when the transfer medium passes a transfer nip between the photoconductive drum (image bearing member) and the transfer roller. When a transfer bias having a polarity opposite to that of toner is applied by the bias power supply, the toner image on the photoconductive drum is transferred to the transfer medium by a transfer electric field. Further, a cleaning member for removing the toner residual on the surface of the photoconductive drum after the image transfer is disposed downstream of the transfer nip in a rotating direction of the photoconductive drum.
In the case of using an amorphous silicon (a-Si) photoconductor as an image bearing member, a surface of the photoconductor is positively charged by a charging roller and an electrostatic latent image after exposure undergoes reversal development with positively charged toner. In a subsequent transfer process, the toner image is transferred to a transfer medium by applying a negative bias having a polarity opposite to that of the toner to a transfer roller.
In the case of using an amorphous silicon photoconductive drum, an output current of a transfer bias needs to be increased to obtain a necessary transfer electric field since a resistance value or a capacitive component of a photoconductive layer is small in relation to the negative transfer bias. Particularly, an output current is set to be relatively high for a transfer medium of a size with a short width since a ratio of a part of the transfer roller directly in contact with the photoconductor is large as compared with the case where a transfer medium has a large width.
Amorphous silicon drums are suitable for a long life and incorporated in high-speed and high-durability machines since it has a high surface hardness and is difficult to abrade. Thus, the transfer roller is required to have a small resistance variation and a good durability even in such a use environment where a large current flows. For example, a foam sponge roller of the electron conductive type obtained by dispersing carbon in an EPDM as a base polymer to provide conductivity is used as a transfer roller having a high durability and a small resistance variation even if a large current bias is applied. In this transfer roller, a volume resistance value is preferably about 7 to 7.5 log Ω. In view of resistance stability, the dispersion amount of the carbon needs to be increased, wherefore the rubber hardness of the transfer roller is consequently about 35 degrees or higher.
On the other hand, in a transfer roller of the ion conductive type, there are problems that a resistance variation in a use environment (temperature and humidity) is large and resistance increases due to the application of a large current of a transfer bias. If the rubber of the transfer roller has a low hardness, the transfer roller may be abraded after a long-term use and, in such a case, problems such as a skew, a magnification defect and a transfer deviation may possibly occur.
Thus, if a transfer roller has a high durability in an image forming apparatus including an amorphous silicon drum, the rubber hardness thereof exceeds 35 degrees in many cases.
Generally, there is often a speed difference (4% to 6%) between a photoconductor and a transfer roller in an image forming apparatus for directly transferring a toner image to a transfer medium using a photoconductive drum. This is to maintain a transfer medium conveying speed in a nip between the photoconductive drum and the transfer roller against a conveyance load of a pre-transfer guide. Thus, the transfer roller is likely to vibrate and to be separated from the photoconductive drum due to this vibration, for example, if transfer press is set to be low or frictional forces between the transfer roller and the photoconductor or the transfer medium are large.
As shown in FIGS. 8A and 8B (same reference numerals as in embodiments are given), a driving force of a transfer roller 10 is input from a transfer gear 55 mounted on one end of a rotary shaft of the transfer roller 10. Depending on a speed difference and a frictional force between a photoconductive drum 7 or a transfer medium S and the transfer roller 10, a driving force from a drive gear 57, transfer load setting or the like, a vibrating state (escaping motion from the drum) of the transfer roller 10 differs. For example, if a frictional force between the photoconductive drum 7 (or transfer medium S) and the transfer roller 10 becomes larger, a torque of the drive gear 57 for driving the transfer roller 10 increases and a driven side at one side of the roller shaft, to which the torque is input, comes to more easily escape (see FIG. 8B). Accordingly, in load setting of the transfer roller 10, a load is generally larger at the side of the drive gear 57.
A first problem of the above construction is as follows. In the case of using the transfer roller 10 having a relatively high rubber hardness, it is assumed that a transfer medium S having a width shorter than a longitudinal dimension of a rubber part of the transfer roller (i.e. narrow sheet) is passed through. In this case, as shown in FIG. 8A, air gaps “a” are formed between the transfer roller 10 and the photoconductive drum 7 due to the thickness of the transfer medium S and the vibration (escaping motion) of the transfer roller near (1 to 2 mm) the opposite end edges of the transfer medium S in a part where the transfer roller 10 and the transfer medium S are in contact. This results from the inability of rubber elasticity to follow the air gaps due to the high rubber hardness of the transfer roller. Thus, there are cases where the surface of the photoconductor is destroyed and black dots appear in a transferred image in long-term use.
A second problem is as follows. As described above, the driving force of the transfer roller 10 is input to the one end of the rotary shaft of the transfer roller and a biasing force of the transfer roller 10 is larger at the drive gear side and smaller at a non-driven side. In this way, it has been tried to make a contact pressure between the transfer roller and the photoconductive drum uniform. However, since the frictional force between the photoconductor or the transfer medium and the transfer roller changes due to various conditions, either one of the opposite ends in a longitudinal direction more easily escapes in many cases. Thus, a large air gap “b” may be formed at one end side and a small air gap “c” may be formed at the other end side as shown in FIG. 8B.
When a transfer medium of a small size is passed through, a surface friction coefficient in sheet non-passage areas of the photoconductive drum 7 is likely to be higher due to the influence of the adhesion of ozone products caused by the discharge of the transfer roller and the absence of surface polishing by the transfer medium. Thus, the frictional force in a contact part with the transfer roller 10 becomes larger and the driven side more easily escapes (air gaps c>b). Conversely, when a transfer medium of a large size is passed through, the transfer roller and the photoconductive drum are not in contact, wherefore the non-driven side more easily escapes (air gaps b>c) if the frictional force is smaller as compared with the case where the transfer medium has a small size. However, this condition changes depending on the surface μ of the transfer medium.
If the transfer press is excessively increased, problems such as polishing nonuniformity of the surface of the photoconductive drum 7 and a hollow phenomenon at the time of passing a thick sheet occur. Accordingly, it is difficult to prevent the vibration of the transfer roller only by the transfer load setting. In this case, a voltage to be discharged in a clearance increases in a state of a large transfer bias output and if this voltage exceeds a withstanding voltage of the drum, a photoconductive layer is destroyed. As a result, electric charges can be no longer retained on the surface of the photoconductor, whereby black dots appear in an image.